Ever since the mechanical properties of composite materials, as for example, glass, polymers, or graphite fibers in a polymer matrix have surpassed those of aluminum, the trend has been for the increased use of these materials in the aerospace industry. Their light weight combined with high strength naturally lends itself to use in aircraft and spacecraft. There has not been, however, widespread development of all composite airplanes because the electrical conductivity of these composite materials is too low to withstand the effects of lightning strike.
As a solution to this problem, metal has been added to the composites. This can take the form of metal surfaces such as metal foils or expanded metal foils, embedded metal wires, and metal filaments, either on the individual composite fibers or on the complete structure.
For example, U.S. Pat. No. 4,448,838 to McClenahan et al. teaches a graphite fiber reinforced laminate structure which contains a metal wire or a metal coated filament which is woven in at least the outer ply of the graphite reinforced laminate structure. The metal provides the graphite reinforced polymer with lightning protection. U.S. Pat. No. 4,481,249 to Ebneth et al. discloses carbon filaments, fibers and sheets manufactured from them which are obtained when the carbon filaments and fibers are provided with a metal coating.
The inclusion of metal in composite structures, however, is not without problems. In instances where metal coatings are used, there is a particular problem with the coatings adhering to the surfaces. As temperatures change, metal layers expand much more rapidly than the glass or carbon fiber layers below. This causes internal stress which is relieved by the separation of the metal layer from the adjacent fiber layer. Additionally, since metals are more dense than the fibers, as more protection is required, more weight is added to the structure. Thus, at some point the advantages of using composite structures are lost because of the added weight. Moreover, the processes by which the metals are included in or on the composite structures are often expensive and labor intensive because adding the metal introduces manufacturing complexities. There also may be complex electrical and chemical interactions between the metal and the organic composite. Often this requires the rigorous exclusion of water from the manufacturing system.
In order to eliminate disadvantages of using metals in composites, the use of metals should be avoided. Therefore, materials other than metals which are good conductors of electricity are needed.
Carbon fibers currently used in the aerospace industry are reasonably good conductors (resistance of 2000 .mu..OMEGA.-cm) but have a resistance which is still nearly three orders of magnitude greater than aluminum (2.7 .mu..OMEGA.-cm). Graphitic carbon is substantially more conductive than the carbon fibers, and graphitic fibers have been made which have resistances as low as 70 .mu..OMEGA.-cm.
The resistance of graphitic carbon fibers has been lowered to values less than that of silver through the process of intercalation, the addition of guest atoms or molecules into the graphite lattice, forming a compound which consists of carbon layers and intercalate layers stacked on top of each other. U.S. Pat. No. 4,414,142 to Vogel et al. teaches the incorporation of intercalated carbon or graphite into an organic composite matrix, resulting in composites with electrical conductivity higher than similar products made with non-intercalated graphite. U.S. Pat. No. 4,749,514 to Murakami et al. discloses a high grade graphite fiber obtained by heat treating specific polymer films and intercalating them in a vacuum or in an inert gas atmosphere. This intercalated graphite has improved stability over prior graphite intercalates. U.S. Pat. No. 4,915,925 to Chung discloses a method for producing exfoliated graphite fibers, by intercalating fibers and then exfoliating the intercalated fibers by heating. These fibers have a high degree of electrical conductivity.
Intercalated pitch based carbon fibers, although having an overall decreased electrical resistance, fail to stand up to lightning strikes. This is apparently caused by the inability of the pitch based fibers to withstand the thermal shock accompanying a lightning strike. U.S. Pat. No. 4,863,773 to Rousseau et al. discloses a composite material of a substrate carbon fiber covered with a fine silicon carbide coating and embedded in a carbon matrix. This material is useful to withstand high heat and is used in spacecraft but does not appear to have improved electrical conductivity.